When large volumes of gaseous oxygen are required for a particular use, the gaseous oxygen is produced by the cryogenic rectification of air in a cryogenic rectification plant and piped directly from the plant to the use point. An air separation plant is designed to operate most efficiently at a certain steady state condition. However, the use point may require the gaseous oxygen under conditions of widely fluctuating demand.
In order to accomodate the countervailing requirements of the efficient steady state operation of the cryogenic rectification plant and the widely fluctuating gaseous oxygen demand of a use point, gaseous oxygen tanks are employed to store gaseous oxygen produced during periods of slack demand and from which gaseous oxygen may be withdrawn and passed to the use point during periods of high demand, thus serving to dampen operating rate fluctuations of the cryogenic air separation plant and thus maintain a high operating efficiency for the plant. A problem with such a system is that even though the gaseous oxygen is stored at high pressure, only a limited amount of gaseous oxygen may be stored in this manner without engaging a gaseous oxygen tank farm which would entail very high capital costs.
The limited storage capacity of backup oxygen may be overcome by storing the oxygen as liquid rather than gas. However, while solving the limited storage problem, this procedure has problems of its own. One problem is that removal of excess oxygen as liquid from the cryogenic rectification plant to be put into storage imposes a large refrigeration loss on the plant. Another problem is that maintaining the stored oxygen in liquid form requires energy input into the system, although this Problem is relatively minor in well insulated tanks. Still another problem is that further energy input is required to vaporize the liquid oxygen to form gaseous oxygen product.
Accordingly, it is an object of this invention to provide an improved cryogenic rectification system for producing gaseous oxygen which can more effectively employ liquid oxygen storage to alleviate or dampen fluctuations in a cryogenic rectification plant operating rate while still accommodating widely fluctuating usage demand for product gaseous oxygen.